Process and product for plating on cast,malleable,carburized and carbonitrided irons



United States Patent PROCESS AND PRODUCT FOR PLATING ON CAST, MALLEABLE,CARBURIZED AND CARBONITRIDED IRONS Edward B. Saubestre, Hamden, andTheophil J.

Wieczorek, West Haven, Conn., assignors to Enthone, Incorporated, NewHaven, Conn.

No Drawing. Filed Mar. 15, 1965, Ser. No. 439,987

6 Claims. (Cl. 20438) ABSTRACT OF THE DISCLOSURE Process for cyanidezinc electroplating ferrous metal surfaces having discrete carboniferousparticles in their surfaces and which enables a sufficiently highhydrogen overvolatge to be maintained during the electroplating to allowthe zinc electroplating to occur. The process involves immersing theferrous metal surface in an aqueous acid solution comprising about 2 to36 ounces per gallon of phosphate ion, 0.15 to 3 ounces per gallon of atleast one wetting agent from the group of nonionic and cationicsurfactants, and about 4 to 20 ounces per gallon of a miscible solventfrom the group of compounds of the formulae:

wherein R is H, -CO-CH -PO(OH) or --SO (OH), R is H or H(CH n is 1-2 andm is 1-4, and optionally an organic thio compound as corrosioninhibitor, the aqueous acid solution having a pH of up to 3.0 inclusiveand being at room temperature or an elevated temperature up to 180 F.,for a time sufficient to form the ferrous surface a thin continuousphosphate-containing film. The thus-coated ferrous metal surface is thenzinc electroplated in a cyanide zinc electroplating bath.

The present invention relates to an improved process for plating on,cast, malleable, carburized and carbonitrided irons and to theresulting product.

The general term cast iron includes gray irons, white cast irons,chilled (white face) cast irons and malleable irons. Cast irons arealloys of iron, carbon and silicon in which more carbon is present thancan be retained in solid solutions in austenite at the eutectictemperature. The carbon content of cast irons is generally 1.5 to 4.5percent. Gray iron is the most widely used of the cast irons.

The term gray iron covers a series of eutectiferous alloys that offer awide selection of mechanical properties, with the composition andprocessing so adjusted that the matrix structure is largely pearlite (alamellar mixture of ferrite and cementite) with many graphitic flakesdispersed throughout. This presence of graphite flakes imparts thecharacteristic gray fracture of these alloys. As a sub-group, we mayconsider the austenitic cast irons, which contain sufficient amounts ofalloying elements to lower the eutectoid transformation temperature tosuch an extent that austenite is retained as the matrix at roomtemperature, with graphite flakes dispersed throughout the structure.(Nickel is commonly used for this purpose.) 'In white cast iron, almostall of the carbon is in the combined form. The presence of ferrite andfree cementite is accompanied by only very Patented Nov. 19, 1968 smallamounts of graphitic matter. Malleable cast iron refers to White castirons which have been heat treated so as to decompose most of thecementite into ferrite and free (or temper) carbon which is usually inthe form of nodularized graphite particles.

Carburizing is the process of increasing the carbon content of theferrous surface by exposing it at high temperature to an atmosphere ofCO +CO with or without hydrocarbon gases so that when quenched, thesurface portion thus carburized will be substantially harder than theunderlying metal. A typical carburized steel may have 1.5 percent C inthe surface layers. The combined proc esses of carburizing and hardeninghave long been known as case hardening. Carbonitriding is that specificexample of carburizing in which ammonia and hydrocarbon gases aredecomposed to provide simultaneous addition of carbon and nitrogen tothe surface being case hardened. Another example of a similar operationis cyaniding, in which molten potassium cyanide is decomposed to achievethe same ends.

In all of the cases described above, the electroplater is faced with asimilar problem: the surface of the metal to be plated contains freecarbon, generally in the form of graphite. The production of asatisfactory plate on cast irons depends upon a high cathodic hydrogenovervoltage on the iron being plated. The graphite or free carbon on thesurface presents two problems; it leads to undissolved smuts on thesurface of the part to be plated in the conventional cleaning cyclesfrequently employed prior to plating and it lowers significantly thehydrogen overvoltage at the site of such graphitic inclusions. Thelatter effect can be quite serious when plating such objects with zincfrom cyanide solutions. In such solutions, standard electrode potentialsfavor the cathodic reduction of hydrogen rather than zinc. The onlyreason that zinc can be plated at all from cyanide solutions is that onmost substrates the hydrogen overvoltage is large, while the zincovervoltage is negligible, thus permitting the cathodic reduction ofzinc preferentially. As noted above, however, surfaces containing freecarbon, especially in graphitic form, greatly lower hydrogen overvoltageso that zinc and other metals do not platedirectly on such surfaces fromcyanide electrolytes.

Free carbon and graphite deposits occur to a large extent in ordinaryuntreated cast irons and in other cases are produced by heat treatment,such as case hardening to improve the strength and resistability of theproduct. In either case, discrete particles of carbon and/or graphite onthe surface either occurring naturally or by the processing of the ironby carburizing or carbonitriding lower the hydrogen overvoltage valuesto such an extent that the evolution of hydrogen and deposition of acontinuous plate on the iron surface is either severely affected orprecluded completely.

ASTM Recommended Practice B320-60 summarizes current thinking regardingmethods for preparation of malleable, gray, nodular and white ironcastings for electroplating. The preparation cycle recommended by ASTMinvolves four basic steps:

(1) Removal of oils, greases, residual polishing and bufiing compoundsand shop dirt by cleaning.

(2) Removal of oxide films and scale and the loosening of surface carbonby pickling or by salt bath treatment.

(3) Removal of any surface smut caused by step (2).

(4) Activation of the casting for subsequent plating. For racked parts,ASTM recommends the following cycle:

(1) Soak cleaning,

(2) Rinse,

(3) Anodic cleaning,

(4) Rinse,

(5) Acid Pickling,

(6) Rinse,

(7) Anodic cleaning,

(8) Rinse,

(9) Neutralization (activation) in 10 percent H 80 if acid plating is tofollow. If alkaline or cyanide plating is to follow, this step iseliminated.

(10) Rinse.

The critical step is the acid pickling step. If the subsequentelectroplating is to be done under conditions causing sufliciently highhydrogen overvoltage (most acid solutions, and such alkaline solution ascopper, cadmium, or tin), a brief dip (less than seconds) in a roomtemperature solution of percent (vol.) HCl (37 percent by weight) or 5to 10 percent (vol.) H SO (98 percent by weight) is usually adequate. Ifthe plating is to be done in an alkaline solution of low hydrogenovervoltage such as cyanide zinc, however, anodic treatment in acid toremove surface carbon is preferred. This is done by making the part theanode in a solution of to percent (vol.) sulfuric acid (98 percent byweight) for at least 30 seconds, preferably more, at 100 a.s.f. While ablack film of carbon smut will form during the first 15 to 30 seconds,the part will become passive and the oxygen evolved at the part willremove the carbon by a combination of scrubbing and oxidation, leavingthe casting relatively clean. Even then, plating may be incomplete incyanide type zinc baths, so that ASTM recommends a preliminaryelectrolytic strike in acid zinc, cyanide cadmium, or alkaline tinbaths.

While not an integral part of this disclosure, it should be noted thatthe inventors have claimed for some time that the above cycle could besignificantly improved by substituting the use of typical alkalinedescalers for the acid pickling step recommended by ASTM. For example,in step 5 of the above cycle, we have previously recommended the use ofa solution containing, for example, about 10 oz. per gallon of an ironchelate, such as EDTA (ethylenedia-minetetraacetic acid), gluconate,oxalate, citrate, heptogluconate and the like, 22 oz. per gallon ofcasutic soda or potash, and 16 oz. per gallon of cyanide of soda orpotash. We have recognized that such solutions may be efiectively usedat room temperature and up to 140 F. using periodic reverse current, 5seconds anodic, 9 seconds cathodic, as an example, for a sufiicientperiod of time, 5 minutes is exemplary to clean the surface to beplated. The use of such materials, in the manner prescribed above,minimizes both attack on the substrate, and the raising of carbon smutsthereon. Generally speaking, it may be said that parts treated by thisrevised procedure plate in a brighter and more uniform manner than thoseplated by the standard ASTM procedure. Nonetheless, even using theseimproved methods, we recognized that in many cases, substrates of thetype discussed above would still yield inferior quality electrodeposits,due to smuts left behind after cleaning, but most seriously might plateincompletely (especially in low current density areas) in cyanide zincsolutions due to the overvoltage phenomenon previously referred to.

We have discovered an improved procedure for preparing carbon andgraphite-containing ferrous surfaces for electroplating, and morespecifically, for preparing such surfaces for electroplating with zincand other metals from cyanide-containing electrolytes without use ofprior electrolytic striking steps, as suggested by ASTM. This inventionthen reveals a method whereby such carbon and graphite-containingferrous surfaces may be directly plated in a cyanide Zinc el ctroyl e Wiout smuts being present on the surface to be plated, and with hydrogenovervoltage in the plating bath so controlled that both throwing andcovering power of the zinciferous electrolyte will be maximized relativeto any previously known methods of preparation of such surfaces for suchplating. It is common commercial practice to cadmium plate cast irons,etc. to avoid the difiiculties of zinc plating such substrates when asacrificially protective coating is required. Since cadmium is bothscarce and expensive, it follows that it is a further object of thisinvention to reveal a more economical method for plating such substrateswth a sacrificially protective corrosion resistant coating. Finally insome cases, cadmium plating is restored to for solderability purposes,since zinc coatings are not readily solderable. It has been noted inprevious patents (US. 2,884,350 and 2,898,274) that it is possible toproduce sacrificially protective readily solderable zinc alloy coatingsfrom cyanide electrolytes. However, cast irons, etc. do not plate wellin the baths of the cited patents either, using previously knownpreparation methods. Hence, a final purpose of the invention is toreveal an improved method of preparing cast iron surfaces whereby suchsubstrates may be coated conveniently with a readily solderable,sacrificially protective corrosion resistant coating using the baths ofUS. 2,884,350 and 2,898,274, for example.

We have found that, quite contrary to normal practice, the improvedmethods which are the subject of our invention result in the formationof a thin continuous chemical film on the part to be plated. This thinchemical film overcomes the initial problem of low hydrogen overvoltagewhen commencing to plate in a cyanide zinc electrolyte. Conventionaldeoxidizing and preparation methods, by contrast, normally aim at makingthe surface to be plated as film-free as possible. We have also foundthat by addition of suitable wetter-s to the filmforming bath it ispossible to sweep away all graphiticbearing particles from the surface,permitting a uniform formation of the desired film.

The film which we have found to be so desirable is a phosphate filmproduced from an aqueous acidic deoxidizing solution containingphosphate ions, together with nonionic, cationic surfactants or both.The film-forming solution may be used in one of three ways in thepreparation for plating procedure for the substrates which are thesubject of our invention. First, it may be used after cleaning as thesole deoxidizing step prior to plating, in which case the disclosedsolution acts both to deoxidize and to form the desired film. Secondly,in the case of heavily oxidized substrates, it may be used aftercleaning and acid pickling such as recommended in the ASTM procedurepreviously noted. Thirdly, and optimally, it may be used followingalkaline descaling as described in the recently improved methods notedabove.

The use of film-forming materials containing phosphates, surfactants andvarious other additives has previously been employed as a pre-cleaningstep followed by the usual conventional methods for preparing such ironsubstrates for subsequent plating. These prior art procedures involvethe immersion of the iron article in an acidic solution containingphosphates and the part is further treated in either an alkalinedescaler or a combination of alkaline electrocleaners. Such materials,however, are completely removed from the surface by a pickling treatmentor otherwise prior to the application of the usual plating procedures tothe film free surface of the iron.

The prior art applications of such film-forming baths did not result ina surface which could be satisfactorily electroplated, particularly tothe application of cyanide zinc plate to the ferriferous surface of thetype which are the subject of this invention. As entirely distinguishedfrom such prior art procedures, the present invention involves theapplication of a thin continuous film to the surface of the iron to beplated which remains in place on the ferriferous surface and the metalfrom the electrolyte is deposited directly on the thin filmed surface ofthe iron. Surprisingly, the result is that high overvoltage ismaintained and a remarkably attractive and firmly adherent electroplateresults.

As indicated, in addition to acid and phosphate ion, the film-formingsolution contains one or more nonionic or cationic surfactants whichwill preferentially wet the surfaces being prepared for plating.Optionally and in addition to the wetters or surfactants, it may beadvantageous in the case of substrates containing unusually largeamounts of graphitic-bearing particles on the surface to add solvents tohelp sweep away all particles from the surface. These solvents should bemiscible in all desired proportions with the acidic medium used.Examples of such solvents are ethylene glycol and many ester and etherderivatives thereof and various ketones.

Optionally and in cases where extensive amounts of deoxidation of thesubstrate are to be performed in the filmforming solution, therebyrequiring long immersion times in the solution, it may be advantageousto add iron corrosion inhibitors to minimize attack on the substrate,thereby also minimizing the amount of smut raised on the surface. Commonexamples of such inhibitors are organic thio-compounds.

In essence, the invention disclosed and claimed herein comprises a castiron product, whose surface contains discrete carboniferous particlesand a relatively thin continuous phosphate-containing pre-plate film,essentially covering a surface of said product and a metal plate firmlyadherent to said pre-plate film. It has been found that, contrary toexpectations, this pre-plate film precludes the depression of thehydrogen overvoltage normally due to the presence of carboniferousparticles on the surface of the cast iron which interferes with orentirely prevents the formation of a continuous and satisfactory plateon the article.

Although the invention is particularly applicable and will be describedwith respect to the electroplating of metals, particularly zinc, it isequally applicable to the electroless deposition of metals on ironsurfaces, pretreated in the indicated manner and has been found togreatly reduce the corrosiveness of the resulting products. The thincontinuous pre-plate film preferably is produced from a solution of aphosphate ion containing material and a surfactant substance selectedfrom the class con sisting of nonionic and cationic surfactants.

As will appear hereinafter, the invention also contemplates a processfor the application of this pre-plate film to the cast iron surface andthe production of an electroplating film directly over and firmlyattached to the thin phosphate containing layer.

Preferably, the pre-plate film created on the iron surface is formedfrom an aqueous solution which comprises from about 2 parts to 36 partsof the phosphate ion and from about 0.15 part to 3.0 parts of thesurfactant substance. Optionally, the film-forming solution may containmiscible solvents, especially in the case of substrates containingunusually large amounts of carbon bearing particles, in an amount offrom about 4.0 parts to 20.0 parts based upon the phosphate surfactantcontent of the solution.

Also optionally the film-forming solution in cases where extensiveamounts of deoxidation of the substrate are to be performed by thefilm-forming solution, it may be advantageous in order to reduceunusually long immersion periods to add iron corrosion inhibitors tominimize attack on the substrate in an amount of from about 0.01 part to1.0 parts also based upon the phosphate surfactant content of thepreparation.

In order to deoxidize effectively the surface of the substrate to beplated, the film-forming solution should be acidic in nature, of a pHranging from between 0.0 and 3.0 and optimally 0.6 to 0.8. It shouldalso contain 2 to 36 ounces per gallon of the phosphate ion (POoptimally, it should contain 10 to 16 ounces per gallon of this ion. Thesimplest way to provide both the required acidity and the requiredamount of phosphate ion is to use phosphoric acid in the film-formingsolution. In terms of the figures above, this would correspond to theuse of 2.5 to 44 ounces per gallon (optimally 12 to 20 ounces pergallon) of 85 percent (wt.) phosphoric acid, as commercially available.Alternatively, but not preferably, the required acidity could beprovided by the use of an acid other than phosphoric (such as sulfuric),combined with the use of a soluble phosphate salt. The salts which maybe used are the orthophosphates, monohydrogen phosphates and dihydrogenphosphate (PO4 3, HPOK H2PO4 respectively) of sodium, potassium andammonium ion. In addition, it is within the scope of this disclosure toinclude as a source of the required phosphate ion free acids and thesodium, potassium and ammonium salts thereof of other forms of thephosphate ion, such as pyrophosphoric (P 0 metaphosphoric (POhexametaphosphoric ((PO tetraphosphates (P O and tripolyphosphates (P OYet another source of the required phosphate ion is the use of organicphosphates, which, generally speaking, are phosphate esters. Thefollowing general reaction shows how phosphoric acid may be readilyesterified with organic bases:

where R is an organic radical. The above equations relate to the acidform of the phosphate esters, but for purposses of this disclosure, itshould be considered that we will be tacitly referring as well to thesodium, potassium, and ammonium salts thereof. Therefore, we will writeRO-PO(OM) for 1 esters, (RO) PO(OM) for 2 esters, where M is H+, New, orNH ion. For reasons of high cost, solubility, and viscosity of solution,if organic phosphates are to be used as a source of the phosphate ion,they should be used in conjunction with additional amounts of phosphateion from inorganic phosphates, unless operating at the lower end of thephosphate ion range noted previously.

For 1, 2 and 3 esters as above defined, R may be either CH or C H For 1esters only, R may further be H(CHOH) CH where n is 1 or 2.

Next, these film-forming solutions must contain surface-active agents,designed to wet preferentially all surfaces being prepared for plating.These may be present in an amount of 0.15 to 3 ounces per gallon;optimally 0.4 to 0.6 ounce per gallon. For reasons of solubility andcompatibility, we generally prefer cationic or non-ionic surface-activeagents (the former of the quaternary ammonium type, the latter of thepoly (oxyethylenated) alkyl aryl type). More specifically, we generallyprefer nonionic agents of the following general formula:

H (CH -Ar-O(CCH CH -O l-I where n is 6 to 20, m is 8 to 12 and Ar is anaryl ring of the phenyl or naphthalene type, and cationic agents of thefollowing general formula:

The following are more specific examples of the general formula above.

where p:1 to 20, X=-H, CH where p=1 to 3 (only), X also X=C H or C l-Iwhere 21:7 to 17 (odd only).

wherep=l to 3 and X=-H,

where X and X are as in II, IIA, HB.

where X X and X are as in II, IIA, HB.

8 In types I and II, R R and R are --H, -OH, CH;;, CH CH CH CH OH,

A- is Cl, Br, Ac, P0 H60 1 Where R=OH, A may also be OH.

III. Heterocyclic amines 0 Hz-- 0 H2 R R CH2CH2 where X is C H or C Hn=8 to 18 (even only). A is as before.

IIIB

IIIA

where X is as in IIIA; R is H or CH A- is as before.

where X is C H or C H 11:7 to 17 (odd only); R is H or -CH A- is asbefore.

IIIE

X-N A- where X and A- are as in IIIC.

where X and A are as in IIID.

In addition to the above types, it has been found by us that it may beadvantageous to combine the need for a surface-active agent with theneed for phosphate ion (at least in part) by the use of phosphate-estertype surfactants. These surfactants are available as 1, 2 and 3 esters,in the same manner as noted above for the use of lower molecular weightorganic phosphate esters:

In these formulae, M stands for H+, Na K+, NH,+ and R stands for anorganic radical. While we have found that a number of such organicradicals are satisfactory for this purpose, we have obtained goodresults using agents in which R is a poly (oxyethylenated) alkyl aryl ora poly (oxyethylenated) aliphatic alcohol. These preferred types are ofthe following two general structures:

H CH -Ar{OCH CH equals R as above defined and where n is 6 to 20, m is 8to 10 and Ar is the phenyl or naphthalene group; and

equals R as above defined and where n is 6 to 20, m is 9 to 13.

The solvent which is an optional part of the film-forming solution maybe of either or both of two general types. The first type consists ofketones of the general structure CH3COR, WhCI'C R CH3-, C2H5, 01'

The second type consists of ethylene glycol ethers and/ or esters of thefollowing general structure where n=1 or 2; R is H or an ester linkageof the following types: -COCH PO(OH) or SO (OH); R is H or an etherlinkage of the type H(CH where 111:1-4. The total amount of combinedsolvents present should be 4 to 20 ounces per gallon; optimally, 8 to 12ounces per gallon. Examples of solvents of the above type are:

Ketones:

Acetone (CH COCH Methyl ethyl ketone (CH COC H Diacetone alcohol In theabove tabulation, the words Cellosolve and Carbitol are registeredtrademarks of Union Carbide. In addition, the correspondnig esters ofeach of the above is to be considered. As a typical example, this wouldbe the formula for ethyl Cellosolve phosphate;

It should be noted that in the event that the particular glycol etherester used is the corresponding phosphate ester, part of the phosphaterequirement of our invention is correspondingly satisfied.

Finally, as a last optional ingredient, the film-forming solution maycontain organic thio-compounds which act as iron corrosion inhibitors inacid solutions. Such materials may be required if the film-formingsolution is used to do an appreciable amount of deoxidizing, leading toextended immersion times of ferriferous substrates in the bath. Whilenot thereby limited, we have found the following compounds to be usefulin this regard: NaSCN;

(C H NCSSNa. These inhibitors are generally used at concentrations ofless than 1 ounce per gallon.

The film-forming solution is generally used at room temperature so as tominimize any attack on the ferriferous substrate, and so as to minimizeheating costs. If desired, however, these solutions may be operated attemperatures up to 180 F. The time of immersion at room temperature isgenerally one to ten minutes, However, it is to be understood that thisdisclosure is not thereby limited, as particular circumstances coulddictate immersion times outside of these ranges.

The following examples represent the preferred embodiments of ourinvention, but are not intended to be inclusive or restrictive in anyway over the above scope. The examples represent actual trials in thelaboratory on parts obtained from industrial plants, as representativeof parts which have been previously impossible to plate directly incyanide zinc electrolytes in a satisfactory manner.

10 EXAMPLE 1 Malleable iron parts which had been previously heat treatedand quenched were rack plated directly in a cyanide zinc bath using thefollowing cycle:

(1) Clean in an alkaline descaler for 5 minutes at room temperature,using periodic reverse current, 5 sec. anodic, 9 sec. cathodic cycle.The bath contained:

Ounces per gallon NaOH 21 Na gluconate ll NaCn 16 (2) Cold water rinse.(3) Immerse in film-forming solution at room temperature for 2 minutes.The bath contained:

Ounces per gallon Glycol ether 8 P0 7 13 Poly (oxyethylenated)nonylphenol 0.6 Diethylthiourea 0.3

pH (electrometric) 0.7.

EXAMPLE 2 Grey iron castings which were rather badly oxidized werebarrel plated directly in a cyanide zinc bath using the following cycle:

(1) Clean in an alkaline descaler for 15 minutes at room temperature,using periodic reverse current, 5 seconds anodic, 8 seconds cathodiccycle. The bath contained:

Ounces per gallon NaOH 26 Na gluconate 6 NaCN 16 (2) Cold water rinse.

(3) Immerse in film-forming solution at room temperature for twominutes. The bath contained:

Ounces per gallon Glycol ether 8 PO4 3 Poly(oxyethylenated) nonylphenol0.6 Diethylthiourea 0.3

pH (electrometric), 0.7.

(4) Cold water rinse.

(5) Barrel zinc plate in a cyanide-type proprietary bright zinc bath forone hour at 7 volts.

(6) Cold water rinse.

(7) Bright dip in chromate dip.

(8) Cold water rinse.

(9) Hot blast dry.

The castings were completely covered even in the lowest current densityrecesses with a uniformly bright zinc deposit.

EXAMPLE 3 Carbonitrided self-locking automotive bolts which were badlyscaled and oiled were barrel plated directly in a cyanide zinc bathusing the following cycle:

(1) Tumble clean in an alkaline heavy-duty soak Ounces per gallon NaOH26 Na gluconate 6 NaCN 16 (4) Double cold water rinse. Immerse infilm-forming solution at room temperature for two minutes. The bathcontained:

Ounces per gallon Glycol ether 16 PO['3 Poly(oxyethylenated) nonylphenol0.6 pH (electrometric) 0.4

EXAMPLE 4 Oxidized cast iron fittings were rack plated directly in acyanide zinc bath using the following cycle:

(1) Clean in alkaline heavy duty soa-k cleaner, used at 8 ounces pergallon, 175 F for 6 minutes, to remove oil.

(2) Double cold 'water rinse.

(3) Immersion film-forming solution at room temperature for 3 minutes.The bath contained:

Ounces per gallon P04 Imidazolinium chloride (quaternary ammonium salt)1 pH (electrometric) 0.4.

(4) Cold water rinse.

(5) Zinc plate in cyanide type proprietary bright zinc bath for minutesat 1 volt.

(6) Cold water rinse.

(7) Bright dip in chromate dip.

(8) Cold water rinse.

(9) Dry.

While we have shown and described some preferred embodiments of ourinvention, it will be understood that it is not to be limited to all ofthe details shown, but is capable of modification and variation Withinthe spirit of the invention and within the scope of the claims. Forexample, the method has been advantageously used in the plating of othermetals such as cadmium, copper, nickel, chromium and electroless nickel.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A process for cyanide zinc electroplating a ferrous metal surface ofan object having discrete carboniferous particles in said surface, whichcomprises immersing the object ferrous metal surface having thecarboniferous particles in said surface in an aqueous acid solutioncomprising about 2 to 36 ounces per gallon of phosphate ion, 0.15 to 3ounces per gallon of at least one lwetting agent selected from the groupconsisting of nonionic and cationic surfactants, about 4 to ounces pergallon of a miscible solvent selected from the group consisting ofcompounds of the formulae:

wherein R is CH;,, C H or whereinR is H,

COCH PO(OH) or SO (OH),

R is H or H(CH n is l-2 and m is 1-4, and an organic thio compound in acorrosion-inhibiting amount less than 1 ounce per gallon, said aqueousacid solution having a pH of up to 3.0 inclusive and being at atemperature in the range of room temperature to 180 F., for a timesufiicient to form thereon a thin continuous phosphatecontaining film,and electroplating a firmly adherent zinc plate onto the thus coatedferrous metal surface in a cyanide zinc electroplating bath whereby asufiiciently high hydrogen overvoltage is maintained during the zincelectroplating to enable said electroplating to occur.

2. The process of claim 1 'wherein the ferrous metal surface having thediscrete carboniferous particles therein is a carburized ferrous metalsurface.

3. The process of claim 1 wherein the phosphate ions are supplied to theaqueous acid solution as phosphoric acid, and the organic thio compoundis selected from the group consisting of sodium thiocyanate,

' 0.15 to 3 ounces per gallon of at least one wetting agent selectedfrom the group consisting of nonionic and cationic surfactants, about 4to 20 ounces per gallon of a miscible solvent selected from the groupconsisting of compounds of the formulae:

ll CH3C-R wherein R is wherein R is H, COCH -PO(OH) or R is H or H(CH nis l2 and m is 1-4, said aqueous acid solution having a pH of up to 3.0inclusive and being at a temperature in the range of room temperature toF., for a time sufiicient to form thereon a thin continuousphosphate-containing film, and electroplating a rlirmly adherent zincplate onto the thus coated ferrous metal surface in a cyanide zincelectroplating bath whereby a sufficiently high hydrogen overvoltage ismaintained during the zinc electroplating to enable said electroplatingto occur.

6. The process of claim 5 wherein the ferrous metal surface having thediscrete carboniferous particles therein is a carburized ferrous metalsurface.

(References on following page) References Cited UNITED STATES PATENTSParker.

Benning et a1. 117134 X 5 Prutton 148-65 Douty et a1. 1486.15 Jernstedt148-615 Dodd et a1. 1486.15 Snyder et a1 148-6.15

14 2,840,498 6/1958 Logue et a1 1486.15 3,133,005 5/1964 Ades 20434 XFOREIGN PATENTS 845,119 5/ 1939 France.

OTHER REFERENCES LoPresti, Metal Finishing, October 1942, pp. 533-536.

RALPH S. KENDALL, Primary Examiner.

